1. Field of the Invention
The present invention relates to a cam follower device which is incorporated in a valve driving mechanism in an engine used to run, for example, an automobile, to reduce the friction occurring in the valve driving mechanism, thereby achieving a reduction in output loss during the running of the engine. More particularly, the present invention pertains to a cam follower device for a valve driving mechanism in an engine, which is designed to achieve improvements in the performance of the engine, particularly an increase in engine speed.
2. Description of the Prior Art
Among various types of engine which are used to run, for example, automobiles, reciprocating engines are all provided with a pair of suction and exhaust valves which are opened and closed synchronously with the rotation of a crankshaft, except for two-cycle engines.
There are various types of valve driving mechanisms for driving the suction and exhaust valves. For example, in an SOHC type valve driving mechanism, which is shown in FIG. 12, a suction valve 4 and an exhaust valve 5 are driven to reciprocate through respective rocker arms 3 by a single cam shaft 2 that rotates at a speed which is half the speed of a crankshaft 1 (in the case of a four-cycle engine). Cams 6 which are rigidly secured to the cam shaft 2 that rotates synchronously with the crankshaft 1, rotate in slide contact with the respective end portions of the rocker arms 3, thereby reciprocatively driving the suction valve 4 and the exhaust valve 5.
Incidentally, it has recently been proposed to provide cam follower devices, which rotate in response to the rotation of the cams 6, in between the cams 6 and the mating rocker arms 3, respectively, to reduce the friction occurring between the peripheral surfaces of the cams 6 and the contact portions of the rocker arms 3 during the running of the engine, thereby achieving a reduction in output loss, and thus improving the engine's efficiency, as disclosed, for example, in Japanese Utility Model Public Disclosure (KOKAI) No. 64-34406 (1989).
More specifically, a cam follower device which is incorporated in an engine for this purpose, is arranged as shown in FIGS. 13 and 14. A pair of spaced support wall portions 7 are provided at the end portion of a rocker arm 3 that faces a cam 6, and two end portions of a shaft 8 are fitted into respective through-holes 11 which are formed in the support wall portions 7, thereby securing the shaft 8 between the pair of support wall portions 7. An outer ring 10, which is in the form of a short cylinder, is provided around the shaft 8 through needle bearings 9. The outer peripheral surfaces of the outer ring 10 and the cam 6 are brought into contact with each other so that the outer ring 10 rotates about the shaft 8 in response to the rotation of the cam 6.
By providing such a rotatable outer ring 10 to change the friction occurring between the cam 6 and a member mated therewith from sliding friction to rolling friction, the output loss during the running of the engine is lowered and fuel consumption decreases, so that the engine efficiency is improved.
There has been another prior art wherein part of the above-described cam follower device is formed from a ceramic material with a view to reducing the overall weight of the cam follower device and improving the high-speed follow-up performance, and thus being suitable in line with the recent tendency for the rotational speed of engines to be increased, as disclosed in Japanese Patent Public Disclosure (KOKAI) No. 63-113108 (1988) and Japanese Utility Model Public Disclosure (KOKAI) Nos. 60-159805 (1985), 62-203911 (1987) and 63-42805 (1988).
Among the conventional cam follower devices of this type, the one disclosed in Japanese Patent Public Disclosure (KOKAI) No. 61-113108 is arranged as shown in FIGS. 15 and 16.
More specifically, a bush 12 which is made of a ceramic material is rotatably fitted around the shaft 8 that is provided between the support wall portions 7 formed at the end of the rocker arm 3, and an outer ring 13 which is similarly made of a ceramic material is fitted around the bush 12 in such a manner that the outer ring 13 is rotatable relative to the bush 12.
In the case of the cam follower device disclosed in Japanese Patent Public Disclosure (KOKAI) No. 63-113108, the ceramic bush 12 is provided between the inner peripheral surface of the ceramic outer ring 13 and the outer peripheral surface of the shaft 8, which is made of steel, thereby lowering the relative sliding velocity between the outer peripheral surface of the steel shaft 8 and the inner peripheral surface of the ceramic bush 12 (in contrast to the arrangement where the outer ring 13 is fitted directly onto the shaft 8), and thus reducing output loss and preventing wear of the outer peripheral surface of the steel shaft 8.
However, a conventional cam follower device for a valve driving mechanism in an engine, such as that disclosed in the above-described Japanese Patent Public Disclosure (KOKAI) No. 63-113108, does not always perform satisfactorily.
More specifically, although the relative velocity between the inner peripheral surface of the bush 12 and the outer peripheral surface of the shaft 8 is lower than in the case where the outer ring 13 is fitted directly onto the shaft 8, it is still impossible to avoid the occurrence of friction therebetween, and no satisfactory reduction in output loss can be achieved. In addition, it is necessary in order to prevent wear of the shaft 8 made of steel, which is softer than a ceramic material, to supply sufficient lubricating oil to a very small clearance that is present between the two peripheral surfaces. The necessary lubrication mechanism accordingly becomes complicated.
On the other hand, Japanese Patent Public Disclosure No. 01-142206 (1989) discloses a cam follower device for an engine, which comprises an outer ring at least the outer surface of which is formed from a ceramic material, a shaft for the outer ring, and a plurality of needle bearings which are interposed between the outer ring and the shaft. By interposing needle bearings between the outer ring and the shaft therefor, it is possible to reduce output loss during the running of the engine and retain an adequate amount of lubricating oil in the area between each pair of adjacent needle bearings. Accordingly, it is possible to effectively lubricate the area between the inner peripheral surface of the outer ring and the outer peripheral surface of the shaft, where the needle bearings are provided.
Although not mentioned in the above-described Japanese Patent Public Disclosure (KOKAI) No. 01-142206, there are many restrictions in practice on the selection of a part of the cam follower device which is to be replaced with a ceramic material. In addition, if a part of the cam follower device is replaced with a ceramic material, very difficult problems arise in combination with other parts which are made of steel.
For example, if one or all of the parts, i.e., the shaft 15, the outer ring 14 and the needle bearings 16, in the structure shown in FIGS. 17 and 18, are made of a ceramic material, it is possible to reduce the inertial mass of the cam follower device correspondingly to the number of parts made of a ceramic material and the mass of these parts and hence cope with the increase in engine speed. However, when the parts 15, 14 and 16 are merely formed from a ceramic material, the following problems arise:
First, when the shaft 15 is made of a ceramic material, if the support wall portions 7 made of either aluminum, which is relatively soft, or a steel, which is plastically deformable, are subjected to staking to secure the shaft 15, the support wall portions 7 cannot bite into the ceramic shaft 15 because it is not plastically deformable. Thus, the support wall portions 7 cannot be effectively staked, and it is therefore difficult to firmly secure the shaft 15 to the support wall portions 7. Accordingly, it has been considered to form the shaft 15 from a steel material so that the end portions of the shaft 15 are plastically deformable, with a view to firmly securing the shaft 15. In this case, the mass increases a little, but the increase in the mass can be minimized, for example, by forming the shaft 15 in a hollow structure. When the needle bearings 16 are made of a ceramic material, the production of the needle bearings 16 becomes difficult, so that the production cost of the cam follower device becomes significantly higher. Since the needle bearings 16 are thin and even the total volume thereof is not large, even if the constitutent material of the needle bearings 16 is changed from a steel material to a ceramic material, the reduction in the weight that is brought about by the change of constituent materials is not large, so that no significant improvement in the high-speed follow-up performance of the cam follower device can be expected. It is therefore preferable to form the needle bearings 16 from a steel material. Under the above-described circumstances, it is concluded that a practically effective way is to form only the outer ring 14 from a ceramic material.
However, the following problems newly arise due to the difference in thermal expansion between the outer ring 14 made of a ceramic material, which has a relatively small coefficient of thermal expansion, and the shaft 15 and the needle bearings 16, which are made of a steel material having a relatively large coefficient of thermal expansion:
When the engine is at rest, the cam follower device is at an ordinary temperature (e.g., 20.degree. C.), whereas, when the engine is in an operative state, the temperature of the cam follower device rises to about 120.degree. C. The thermal expansion of the shaft 15 and the needle bearings 16 that is caused by the rise in the temperature is greater than that of the outer ring 14 made of a ceramic material.
Accordingly, when the temperature of the cam follower device rises as the engine is run, the size of the clearance that is present where the steel needle bearings 16 are disposed, that is, the dimension h that is determined by subtracting the sum of the outer diameter D of the shaft 15 and double the outer diameter d of a needle bearing 16 from the inner diameter R of the outer ring 14, i.e., h=R-(D+2d), decreases. If the clearance h becomes excessively small on such an occasion, the needle bearings 16 may seize. In an extreme case, the ceramic outer ring 14 may be cracked by being forcibly extended outwardly.
If the clearance h at ordinary temperature is set at an excessively large value with a view to preventing the seizure of the needle bearings 16 and possible cracking of the outer ring 14, the level of noise generated from the cam follower device becomes excessively high during the initial running stage of the automotive engine when the temperature of the cam follower device is still low and, in an extreme case even when the temperature of the engine has risen.